Segment Information - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
Sep. 30, 2020 USD ($) OperatingDivisions State Home Market Segments	Sep. 30, 2020 USD ($) State Home Market	Sep. 30, 2019 USD ($) Home	Sep. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Proceeds from Sale of Buildings		$ 129.8	$ 143.8	$ 292.9
Gain (Loss) on Sale of Properties		59.5	53.9	$ 18.8
Assets	$ 18,912.3	$ 18,912.3	$ 15,606.6
Home Building [Member]
Segment Reporting Information [Line Items]
Number of homebuilding operating divisions \| OperatingDivisions	53
Number of homebuilding reporting segments \| Segments	6
Homebuilding percentage of consolidated revenues		97.00%	97.00%	97.00%
Number of housing construction markets \| Market	88	88
Number of housing construction states \| State	29	29
Gain (Loss) on Sale of Properties		$ 0.0	$ 2.0	$ 15.8
Assets	$ 15,221.4	15,221.4	12,715.9
Other Segments [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Sale of Properties		59.4	51.9	$ 0.0
Assets	$ 379.4	379.4	317.9
Other Segments [Member] \| Multifamily [Member]
Segment Reporting Information [Line Items]
Proceeds from Sale of Buildings		128.5	133.4
Gain (Loss) on Sale of Properties		$ 59.4	$ 51.9
Multi-Family Units, Total \| Home	1,730	1,730
Multi-Family Units, Under Active Construction \| Home	1,430	1,430
Multi-Family Units, Completed \| Home	300	300
Unit Sales, Multi-Family \| Home		540	820
Assets	$ 246.2	$ 246.2	$ 204.0
Other Segments [Member] \| Multifamily [Member] \| Asset under Construction [Member]
Segment Reporting Information [Line Items]
Number of Projects \| Home	5	5
Other Segments [Member] \| Multifamily [Member] \| Assets Leased to Others [Member]
Segment Reporting Information [Line Items]
Number of Projects \| Home	1	1
Forestar Group [Member]
Segment Reporting Information [Line Items]
Number of housing construction markets \| Market	49	49
Number of housing construction states \| State	21	21

Homebuilding revenues expressed as a percentage of consolidated revenues.

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The number of markets the entity operates in as of the balance sheet date.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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